1. Field of the Invention
The present invention relates to an objective to be used in endoscopes in which an image of an object is transmitted in turn by a plural number of relay lens groups and, more particularly, to a retrofocus type objective comprising a front diverging lens group and rear converging lens group.
2. Description of the Prior Art
Most of the known objectives for endoscopes comprise a single lens or two or three lenses due to reasons that the distal end of an endoscope is small in diameter and that it is therefore difficult to manufacture the lenses. On the other hand, in endoscopes, the image is transmitted several times by means of relay lens groups and, therefore, it is necessary to arrange the relay lens groups so that their lens surfaces contacting air are minimized in order to reduce the loss of light caused by reflection on lens surfaces contacting air. So, it is unavoidable to arrange the relay lens group so that it has simple lens configuration and, consequently, sagittal astigmatism and meridional astigmatism of negative values are necessarily caused. Moreover, as a plural number of relay lens groups are used in an endoscope, the relay lens system as a whole causes a considerably large amount of astigmatism. Therefore, for the final image obtained by the endoscope, it is impossible to favourably focus the image in the central portion and image in the marginal portion at the same time even when aberrations of the objective are corrected favourably. Besides, in cases of side-viewing endoscopes and oblique-viewing endoscopes, a prism or prisms are arranged in front of the objective in order to refract the light. When, however, the field angle of the objective is large, the prism necessarily becomes considerably large and it becomes impossible to arrange such large prism in the distal end of the endoscope which has a small diameter. Moreover, when the prism becomes large, it is unavoidable to make the cover glass also large.